Impact of biochar amendment on enzymatic resilience properties of mine spoils

Biochar Co-Applied with Lime Enhances Soil Phosphorus Availability via Microbial and Enzymatic Modulation of Paddy Soil

Soil microorganisms play a crucial role in improving soil phosphorus (P) availability. However, few studies have explored the changes in microbial community structure and their underlying mechanisms for improving soil P availability with the application of biochar and lime. Three kinds of biochar, made from rice straw (SB), Chinese fir wood sawdust (WB), and pig manure (MB), alone and with lime (SBL, WBL, and MBL), were applied to paddy soil to reveal the biochemical mechanisms for enhancing soil P availability. High-throughput sequencing and real-time PCR were used to investigate soil microbial communities and P functional genes. The three biochars increased the soil's available P in the order of MB > SB > WB. Biochar co-applied with lime increased the available P (Olsen-P by 169-209%) and inorganic P (Al-P by 53.4-161%, Fe-P by 96.3-198%, and Ca-P by 59.0-154%) more than biochar alone, compared to the control (CK). Both biochar alone and co-applied with lime increased the activities of alkaline phosphomonoesterase (ALP), phosphodiesterase (PD), and inorganic pyrophosphatase (IPP) by 369-806%, 28.4-67.3%, and 37.9-181%, respectively, while it decreased the activity of acidic phosphomonoesterase (ACP) by 15.1-44.0%, compared to CK. Biochar, both alone and co-applied with lime, reduced the copy number of phoC gene by 5.37-88.7%, while it increased the phoD, gcd, and pqqC genes by 51.3-533%, 62.1-275%, and 25.2-158%, respectively, compared to CK. A correlation analysis and partial least squares path modeling (PLS-PM) indicated that Olsen-P, Bray-1 P, and inorganic P were significantly positively correlated with the activities of ALP, PD, IPP, and the phoD gene. Biochar co-applied with lime increased the relative abundances of the phoD-harboring bacteria Proteobacteria, Firmicutes, and Acidobacteria, which promoted the transformation of P to the effective state. Meanwhile, the dominant species Anaerolinea, Ascomycota, Mucoromycota, and Chaetomium provided rich effective nutrients for the soil microorganisms by accelerating the decomposition of soil organic matter, thus promoting phosphatase activity. It could be inferred that the optimized microbial community structure improved phosphatase activity by increasing the phoD gene and available nutrients, thus promoting the soil P availability. Biochar co-applied with lime had a better effect on increasing the P availability and rice yields than biochar alone.

Synergistic biochar and Serratia marcescens tackle toxic metal contamination: A multifaceted machine learning approach

Metal contamination in soil poses environmental and health risks requiring effective remediation strategies. This study introduces an innovative approach of synergistically employing biochar and bacterial inoculum of Serratia marcescens to address toxic metal (TM) contamination. Physicochemical, enzymatic, and microbial analyses were conducted, employing integrated biomarker response (IBR) and machine-learning approaches for toxicity estimation. The combined application significantly reduced the Cd, Cr, and Pb concentrations by 71.6, 31.2, and 57.1%, respectively, while the Cu concentration increased by 85% in the individual Serratia marcescens treatment. Biochar enhanced microbial biomass by 33-44% after 25 days. Noteworthy physicochemical improvements included a 44.7% increase in organic content and a decrease in pH and electrical conductivity. The K⁺ and Ca2⁺ concentrations increased by 196.9 and 21.6%, respectively, while the Mg2⁺ content decreased by 86.4%. Network analysis revealed intricate relationships, displaying direct and indirect negative correlations between metals and soil physicochemical parameters. The IBR index values indicated effective mitigation of TM toxicity in Serratia marcescens and biochar with individual and combined treatments. Binary classification demonstrated high sensitivity (80.1%) and specificity (80.5%) in identifying TM-contaminated soil. These findings indicate significant biochar- and Serratia marcescens-induced impacts on toxic metal availability, physicochemical properties, and enzymatic activities in metal-contaminated soil, suggesting that blending soil with biochar and microorganisms is an effective remediation strategy.

Food waste biochar for sustainable agricultural use: Effects on soil enzymes, microbial community, lettuce, and earthworms

This study investigates the effects of food waste biochar (FWB) on the biological properties of soil, including the microbial community structure, enzyme activities, lettuce growth, and earthworm ecotoxicity. This holistic assessment of various soil organisms was used to assess the potential of FWB as a soil amendment strategy. Pot experiments were carried out over a 28-d period using various FWB concentrations in soil (0-3% w/w). The presence of FWB enhanced the activity of alkaline phosphatase and beta-glucosidase in proportion to the FWB concentration. Similarly, the dehydrogenase activity after 28 d was positively correlated with the FWB concentration. Notably, the application of FWB improved the bacterial diversity in the soil, particularly among hydrocarbonoclastic bacteria, while also prompting a shift in the fungal community structure at the class level. Measures of lettuce growth, including total fresh weight, shoot length, and leaf number, also generally improved with the addition of FWB, particularly at higher concentrations. Importantly, FWB did not adversely affect the survival or weight of earthworms. Collectively, these findings suggest that FWB can enhance soil microbial enzyme activity and support plant growth-promoting rhizobacteria, potentially leading to increased crop yields. This highlights the potential of FWB as an eco-friendly soil amendment strategy.

Driving Factors Influencing Soil Microbial Community Succession of Coal Mining Subsidence Areas during Natural Recovery in Inner Mongolia Grasslands

Soil microorganisms significantly influence the energy flow and material cycle of soil ecosystems, making them highly susceptible to environmental changes, such as those induced by mining activities. Studying the succession of soil microbial communities after mining subsidence is crucial for comprehending the significance of soil microbes in the natural recovery process following subsidence. Therefore, the soil properties, vegetation communities, and soil microbial communities of the subsidence area, as well as unexploited areas, were analyzed during the natural restoration process (1, 2, 5, 10, and 15 years). The results demonstrate that mining subsidence has a significant impact on the aboveground vegetation community, soil properties, and microbiological community. Following an extended period of natural recovery, a new stable state has emerged, which differs from that observed in non-subsidence areas. The total nitrogen, nitrate nitrogen, and ammonium nitrogen amounts may be key factors driving the natural recovery of bacterial communities, and total potassium and available potassium may be key factors driving the natural recovery of fungal communities. The natural recovery mechanism of soil microorganisms was analyzed along with the changes related to vegetation and soil physicochemical properties. The mechanism was explained from three perspectives, namely, plant-led, soil-led, and soil-microbial-led, which could provide a theoretical basis for the natural restoration of grassland ecosystems and provide guidance for the treatment of coal mining subsidence areas.

Bacteria-Virus Interactions Are More Crucial in Soil Organic Carbon Storage than Iron Protection in Biochar-Amended Paddy Soils

Iron oxides supposedly provide physicochemical protection for soil organic carbon (SOC) under anoxic conditions. Likewise, biochar can modulate the composition of soil microbial communities. However, how Fe oxides and microbial communities influence the fate of SOC with biochar amendment remains unresolved, especially the effect of the bacteria-virus interaction on SOC dynamics. Here, we performed a four-month pot experiment using rice seedlings with a biochar amendment under waterlogged conditions. Then, soil aggregate sizes were examined to explore the factors influencing the SOC patterns and the underlying mechanisms. We found that biochar altered soil enzyme activities, especially in macroaggregates. Fe oxides and necromass exhibited significant negative relationships with SOC. Bacterial communities were notably associated with viral communities. Here, the keystone ecological cluster (module 1) and keystone taxa in the bacteria-virus network showed significant negative correlations with SOC. However, Fe oxides exhibited substantial positive relationships with module 1. In contrast to the prevailing view, the SOC increase was not primarily driven by Fe oxides but strongly influenced by bacteria-virus interactions and keystone taxa. These findings indicate that biochar governs microbial-mediated SOC accumulation in paddy soil and ascertains the role of viruses in regulating the bacterial community, thus predicting SOC stock.

Effect of iron-loaded sludge biochar amendments on phytoremediation potential of Cr-contaminated soils by Leersia hexandra swartz

In this study, the effect of iron-loaded sludge biochar (ISBC) with different amendment dosages (mass ratio of biochar to soil equal to 0, 0.01, 0.025 and 0.05) on the phytoremediation potential of Leersia hexandra swartz (L. hexandra) to Cr-contaminated soil was investigated. With increasing ISBC dosage from 0 to 0.05, plant height, aerial tissue biomass and root biomass increased from 15.70 cm, 0.152 g pot-1 and 0.058 g pot-1 to 24.33 cm, 0.304 g pot-1 and 0.125 g pot-1, respectively. Simultaneously, the Cr contents in aerial tissues and roots increased from 1039.68 mg kg-1 to 2427.87 mg kg-1 to 1526.57 mg kg-1 and 3242.62 mg kg-1, respectively. Thus, the corresponding bioenrichment factor (BCF), bioaccumulation factor (BAF), total phytoextraction (TPE) and translocation factor (TF) values were also increased from 10.52, 6.20, 0.158 mg pot-1 (aerial tissue)/0.140 mg pot-1 (roots) and 0.428 to 15.15, 9.42, 0.464 mg pot-1 (aerial tissue)/0.405 mg pot-1 (roots) and 0.471, respectively. The significant positive effect of ISBC amendment was primarily attributed to the following three aspects: 1) the root resistance index (RRI), tolerance index (TI) and growth toxicity index (GTI) of L. hexandra to Cr were increased from 100%, 100% and 0%-216.88%, 155.02% and 42.18%, respectively; 2) the bio-available Cr content in the soil was decreased from 1.89 mg L-1 to 1.48 mg L-1, while the corresponding TU (toxicity units) value was declined from 0.303 to 0.217; 3) the activities of urease, sucrase and alkaline phosphatase in soil were increased from 0.186 mg g-1, 1.40 mg g-1 and 0.156 mg g-1 to 0.242 mg g-1, 1.86 mg g-1 and 0.287 mg g-1, respectively. In summary, ISBC amendment was able to significantly improve the phytoremediation of Cr-contaminated soils by L. hexandra.

Biochar as a Green Sorbent for Remediation of Polluted Soils and Associated Toxicity Risks: A Critical Review

Soil contamination with organic contaminants and various heavy metals has become a global environmental concern. Biochar application for the remediation of polluted soils may render a novel solution to soil contamination issues. However, the complexity of the decontaminating mechanisms and the real environment significantly influences the preparation and large-scale application of biochar for soil ramification. This review paper highlights the utilization of biochar in immobilizing and eliminating the heavy metals and organic pollutants from contaminated soils and factors affecting the remediation efficacy of biochar. Furthermore, the risks related to biochar application in unpolluted agricultural soils are also debated. Biochar production conditions (pyrolysis temperature, feedstock type, and residence time) and the application rate greatly influence the biochar performance in remediating the contaminated soils. Biochars prepared at high temperatures (800 °C) contained more porosity and specific surface area, thus offering more adsorption potential. The redox and electrostatic adsorption contributed more to the adsorption of oxyanions, whereas ion exchange, complexation, and precipitation were mainly involved in the adsorption of cations. Volatile organic compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) produced during biochar pyrolysis induce negative impacts on soil alga, microbes, and plants. A careful selection of unpolluted feedstock and its compatibility with carbonization technology having suitable operating conditions is essential to avoid these impurities. It would help to prepare a specific biochar with desired features to target a particular pollutant at a specific site. This review provided explicit knowledge for developing a cost-effective, environment-friendly specific biochar, which could be used to decontaminate targeted polluted soils at a large scale. Furthermore, future study directions are also described to ensure a sustainable and safe application of biochar as a soil improver for the reclamation of polluted soils.

Biochar as a potential strategy for remediation of contaminated mining soils: Mechanisms, applications, and future perspectives

Soil heavy metal contamination caused by mining activities is a global issue. These heavy metals can be enriched in plants and animals through the food chain, and eventually transferred to the human system and threatening public health. Biochar, as an environmentally friendly soil remediation agent, can effectively immobilize heavy metals in soil. However, most researchers concern more about the remediation effect and mechanism of biochar for industrial and agricultural contaminated soil, while related reviews focusing on mining soil remediation are limited. Furthermore, the remediation effect of soil in mining areas is affected by many factors, such as physicochemical properties of biochar, pyrolysis conditions, soil conditions, mining environment and application method, which can lead to great differences in the remediation effect of biochar in diverse mining areas. Therefore, it is necessary to systematically unravel the relevant knowledge of biochar remediation, which can also provide a guide for future studies on biochar remediation of contaminated soils in mining areas. The present paper first reviews the negative effects of mining activities on soil and the advantages of biochar relative to other remediation methods, followed by the mechanism and influencing factors of biochar on reducing heavy metal migration and bioavailability in mining soil were systematically summarized. Finally, the main research directions and development trends in the future are pointed out, and suggestions for future development are proposed.

Addition of walnut shells biochar to alkaline arable soil caused contradictory effects on CO2 and N2O emissions, nutrients availability, and enzymes activity

Mitigation of greenhouse gas (GHGs) emissions and improving soil health using biochar (BC) shall help achieving the UN-Sustainable Development Goals. The impacts of walnut shells biochar (WSB) pyrolyzed at different temperatures on CO₂ and N₂O emission and soil health have not been yet sufficiently explored. We investigated the effects of addition of WSB pyrolyzed at either 300 °C (WSB-300), 450 °C (WSB-450), or at 600 °C (WSB-600) to alkaline soil on CO₂ and N₂O emissions, nutrients availability, and soil enzymes activities in a 120-day incubation experiment. Cumulative N₂O emissions were reduced significantly as compared to the control, by 64.9%, 50.6%, and 36.4% after WSB-600, WSB-450 and WSB-300, respectively. However, the cumulative CO₂ emissions increased, over the control, as follows: WSB-600 (50.7%), WSB-450 (68.6%), and WSB-300 (73.4%). Biochar addition, particularly WSB-600 significantly increased soil pH (from 8.1 to 8.34), soil organic C (SOC; from 8.6 to 22.3 g kg^-1), available P (from 21.0 to 60.5 mg kg^-1), and K (181.0-480.5 mg kg^-1), and activities of urease, alkaline phosphatase, and invertase. However, an opposite pattern was observed with NH₄⁺, NO₃^-, total N and β-glucosidase activity after WSB application. The WBS produced from high temperature pyrolysis can be used for N₂O emissions mitigation and improvement of soil pH, SOC, available P and K, and activities of urease, alkaline, phosphatase. However, WBS produced from low temperature pyrolysis can be used to promote N availability and β-glucosidase; however, these findings should be verified under different field and climatic conditions.

Ameliorative effects of biochar on persistency, dissipation, and toxicity of atrazine in three contrasting soils

The presence of atrazine a persistent herbicide in soil poses a serious threat to the ecosystem. The biochar amendment in soil altered the fate of this herbicide by modifying the soil properties. The present study examines the dissipation and toxicity of atrazine in three contrasting soils (silty clay, sandy loam, and sandy clay) without and with biochar amendment (4%). The experiment was performed for 150 days with three application rates of atrazine (4, 8, and 10 mg kg^-1). The speciation and degradation of atrazine, metabolite content, microbial biomass, and enzymatic activities were evaluated in all treatments. Three kinetic models and soil enzyme index were calculated to scrutinize the degradation of atrazine and its toxicity on soil biota, respectively. The goodness of fit statistical indices suggested that the first-order double exponential decay (FODE) model best described the degradation of atrazine in silty clay soil. However, a single first order with plateau (SFOP) was best fitted for atrazine degradation in sandy loam and sandy clay soils. The half-life of atrazine was higher in sandy clay soil (27-106 day^-1) than silty clay (28-77 day^-1) and sandy loam soil (27-83 day^-1). The variations in the dissipation kinetics and half-life of the atrazine in three soil were associated with atrazine partitioning, availability of mineral content (silica, aluminum, and iron), and soil microbial biomass carbon. Biochar amendment significantly reduced the plateau in the kinetic curve and also reduced the atrazine toxicity on soil microbiota. Overall, biochar was more effective in sandy clay soil for the restoration of soil microbial activities under atrazine stress due to modulation in the pH and more improved soil quality.

The incorporation of lemongrass oil into chitosan-nanocellulose composite for bioaerosol reduction in indoor air

The bioaerosols present in indoor air play a major role in the transmission of infectious diseases to humans, therefore concern about their exposure is increased recently. In this regard, the present investigation described the preparation of lemongrass essential oil (LGEO) loaded chitosan and cellulose nanofibers composites (CH/CNF) for controlling the indoor air bioaerosol. The evaluation of the inhibitory effect of the composite system on culturable bacteria of the indoor air was done at different sites (air volume from 30 m³ to 80 m³) and in different size fractions of aerosol (<0.25 μm-2.5 μm). The composite system had high encapsulation efficiency (88-91%) and citrals content. A significant reduction in culturable bacteria of aerosol (from 6.23 log CFUm^-3 to 2.33 log CFUm^-3) was observed in presence of cellulose nanofibers and chitosan composites. The bacterial strains such as Staphylococcus sp., Bacillus cereus, Bacillus pseudomycoides sp., Pseudomonas otitidis, and Pseudomonas sp. Cf0-3 in bioaerosols were inhibited dominantly due to the diffusion of aroma molecules in indoor air. The results indicate that the interaction of diffused aroma molecule from the composite system with bacterial strains enhanced the production of ROS, resulting in loss of membrane integrity of bacterial cells. Among different size fractions of aerosol, the composite system was more effective in finer size fractions (<0.25 μm) of aerosol due to the interaction of smaller aroma compounds with bacterial cells. The study revealed that LGEO loaded chitosan and cellulose nanofibers composites could be a good option for controlling the culturable bacteria even in small-sized respirable bioaerosol.

Benefits of Corn-Cob Biochar to the Microbial and Enzymatic Activity of Soybean Plants Grown in Soils Contaminated with Heavy Metals

Synchronous effects of biochar on heavy metals stress, microbial activity and nodulation process in the soil are rarely addressed. This work studied the effects, under greenhouse conditions, of selected heavy metals Cd2+, Pb2+ and Ni2+ on soybean plants grown in two different soils amended with biochar, and studied their effect on the microbial and enzymatic activity. As a result of the interference between heavy metals and biochar, biochar overcame heavy metal problems and maintained a microbial population of major groups (bacteria–fungi). There was an increase in the degree of resistance (RS) of the major microbial groups to heavy metals when biochar was added to the soil under study. Numbers of bacterial nodules significantly increased, particularly by using the higher rate of biochar compared to the control, either by adding biochar alone or by mixing it with the selected heavy metals. The arginase activity was increased by 25.5% and 37.1% in clay and sandy soil, respectively, compared to the control. For urease (UR), the activity was increased by 105% and 83.8% in clay and sandy soil, respectively, compared to the control. As a result, considerations of using biochar as a soil amendment should be first priority.

Use of Piptatherum miliaceum to enable the establishment success of Salvia rosmarinus in Technosols developed from pyritic tailings

With this study we aimed to assess the effect of the prior development of Piptatherum miliaceum (Pm) in a Technosol on the establishment of Salvia rosmarinus (Sr) as a cash crop. An experimental pot was performed with two biochar (BCh) doses (BCh1 and BCh2) mixed with marble waste and pyritic tailings, with and without Pm. After 12 months of Pm growth, the pots with this species were divided into two sets: Sr alone and Sr + Pm. An agricultural soil (AGR) was used as an external control. The results showed that the growth of Sr led to similar shoot biomass to AGR. Sr + Pm reduced shoot biomass by 50%. Total soil organic and recalcitrant C, and total and recalcitrant N showed the highest values in vegetated pots, with no effect of the BCh rate. The decrease in the soil metals availability was related with increases in soil pH. BCh1Sr + Pm treatment showed a microbial community structure more similar to AGR, related to higher fungal and bacterial abundance, enzyme activities and soluble carbon. Thus, growing Sr + Pm seems a suitable strategy to improve soil properties, including microbial abundance, with low translocation of metals. Although the BCh rate did not affect plant growth or soil physicochemical properties, the lowest rate contributed to the growth of soil microorganisms better. The simultaneous growth of Sr + Pm reduced biomass production, and a source of available nutrients is also recommended. Further studies are needed to test this strategy in the field, and to ensure its suitability and a constant biomass production.

Mobilization or immobilization? The effect of HDTMA-modified biochar on As mobility and bioavailability in soil

Biochar plays an essential role in soil remediation, but its effect on the arsenic remediation has been controversial. In this study, hexadecyl trimethyl ammonium bromide (HDTMA-Br) modified or unmodified biochar on As mobility and bioavailability in soil were studied. The sequential extraction experiment showed that As in the original soil mainly existed in the occluded form (78.24%), followed by Fe‒As (20.72%) and Al‒As (0.88%) forms. With the addition of the modified and unmodified biochars, the contents of Ca‒As and Fe‒As increased by 0.36 - 0.95% and 2.06 - 3.36%, respectively, suggesting the increased potential toxicity of As. The NaH₂PO₄ extraction result showed that the unmodified biochar increased the As availability by 3.23 - 22.76%, whereas the HDTMA-modified biochar reduced the As availability by 4.80 - 13.41%. Pot experiment showed that the unmodified and modified biochar increased the biomass of Brassica pekinensis, and the modified biochar (HB5) decreased the uptake of As by plants by 80.77% compared to the unmodified biochar. In particular, the plant achieved better growth in the modified biochar treatment (average height 8.31 cm) than in the unmodified biochar treatment (average height 6.97 cm). Therefore, both biochars facilitated phase transformation of As from the stable to the mobile states in the soil. Nevertheless, the HDTMA-modified biochar had an effect on alleviating As bioavailability and toxicity.

Effect of cornstalk biochar on phytoremediation of Cd-contaminated soil by Beta vulgaris var. cicla L

Cadmium (Cd) contamination is the most common and extensive heavy metal pollution in the farmland of China. Phytoremediation is considered as a promising measure for Cd-contaminated soil remediation, but the remediation efficiency still needs to be enhanced. Biochar as an effective amendment medium is widely manufactured and studied for the soil remediation of heavy metals. In this study, a greenhouse pot trial was conducted to investigate the effects of cornstalk biochar on Cd accumulation of Beta vulgaris var. cicla L. (Beta vulgaris) in Cd contaminated soil. The Cd availability, speciation and nutrients in soil, biomass and Cd chemical forms in the Beta vulgaris root were studied to explore the mechanism that how the cornstalk biochar promoted Cd accumulation in Beta vulgaris. Biochar amendment reduced the DTPA-extractable Cd concentration and stimulated the growth of root. Compared to the Beta vulgaris without biochar treatment, the results of 5% biochar amendment showed that the root dry weight of Beta vulgaris increased to 267%, Cd accumulation in Beta vulgaris increased to 206% and the Cd concentration in leaves and roots increased by 36% and 52%, respectively. Additionally, after 5% biochar was applied to soil, the total content of organic matter-bound Cd and residual Cd increased by 38%, while the content of Fe-Mn oxides-bound Cd decreased by 40%. Meanwhile, Cd may mainly bind to the root cell wall and the ratio of NaCl-extracted Cd to HAc-extracted Cd increased to 166% with 5% biochar amendment. According to our study, Cd in soil can be removed by Beta vulgaris and phytoremediation efficiency can be improved with biochar amendment. The combination of phytoremediation and biochar amendment is a promising strategy for the Cd-contaminated soil remediation.

Can biochar reclaim coal mine spoil?

Surface coal mining activities completely destroy vegetation cover, soil and biodiversity. The aftermaths include huge coal mine spoil dumps, changed topography, drainage and landscape, deteriorated aesthetics and increased pollution load. These coal mine spoils are characterised by high rock fragments, extremely low water holding capacity, compacted and high bulk density, lack of organic carbon and plant nutrients, low cation exchange capacity, acidic pH and toxic metal contamination, which poses difficulties in reclamation. An array of studies has been focused on the sustainable use of biochar for restoration of degraded agricultural soil by improving the soil physicochemical, nutritional and biological properties. Although a volume of studies has been done on biochar application, its specialised application in reclamation of coal mine spoils is still atypical, also a systematic review on the mechanism by which biochar amends the mine spoil is lacking. This review focuses on i) factors affecting the biochar properties, ii) the mechanism involved in altering the physical, chemical and biological properties by biochar, (iii) remediation of potentially toxic elements in soil and restoration of degraded land using biochar, and, iv) highlighting the important aspects to be considered while using biochar for reclamation of coal mine spoil. Biochar prepared at 450 °C from a lignocellulosic rich biomass can be an alternative for reclamation for coal mine spoil. Review also suggested suitable methodologies for bulk production, application and economics of biochar in coal mine spoil reclamation.

Apricot shell- and apple tree-derived biochar affect the fractionation and bioavailability of Zn and Cd as well as the microbial activity in smelter contaminated soil

The aim of this study was to elucidate the effects of apricot shell-derived biochar (ASB) and apple tree-derived biochar (ATB) on soil properties, plant growth, microbial communities, enzymatic activities, and Zn and Cd fractionation and phytoavailability in mining soils. Smelter soil contaminated by Zn (1860.0 mg kg-1) and Cd (39.9 mg kg-1) was collected from Fengxian, China, treated with different doses (0 (control), 1, 2.5, 5, and 10% w/w) of both biochars and cultivated by Brassica juncea in a greenhouse pot experiment. The acid-soluble, reducible, oxidizable, and residual fraction and plant tissue concentrations of Zn and Cd were determined. Biochar addition improved plant growth (22.6-29.4%), soil pH (up to 0.94 units), and soil organic matter (up to 4-fold) compared to the control. The ASB and ATB, particularly ATB, reduced the acid-soluble (21-26% for Zn and 15-35% for Cd) and the reducible (9-36% for Zn and 11-19% for Cd) fractions of Zn and Cd and altered these fractions in the organic and residual fractions. Therefore, the biochars decreased the metal concentrations in the roots (36-41% for Zn and 33-37% for Cd) and shoots (25-31% for Zn and 20-29% for Cd), which might be due to the increase in pH, biochar liming effects, and metal sorption by the biochar. The biochars impact on the bacterial community composition was selective. The ASB and ATB decreased the activities of soil β-glucosidase, dehydrogenase, and alkaline phosphatase while increasing the urease activity. The biochars, particularly ATB, can be considered as effective soil amendments for reducing the phytotoxicity of Zn and Cd in contaminated soils, improving plant growth, enhancing the abundance of specific bacterial groups and increasing urease activity; however, more attention should be paid to their negative effects on the activities of β-glucosidase, dehydrogenase, and alkaline phosphatase.

Biochar aided aromatic grass [Cymbopogon martini (Roxb.) Wats.] vegetation: A sustainable method for stabilization of highly acidic mine waste

Dumping of acidic mine waste poses severe threats to the ecosystem due to high acidity, nutrient deficiency and mobility of toxic metals. The present study has been undertaken on phytoremediation by amending the acidic soil/mine waste with biochar (BC) and plantation of palmarosa (Cymbopogon martini (Roxb.) Wats. A greenhouse experiment in different combinations of biochar and acidic mine waste was conducted to assess the phytoremediation efficiency of palmarosa by BC amendments. Results indicate that the palmarosa tolerates multiple stresses effectively with a 54 % metal tolerance index (MTI) and capable of reducing acid production from the acidic mine waste alone. BC incorporation in the mine waste and soil treatments significantly enhanced the palmarosa biomass (1.11-3.3 times) and oil content by liming the acid, immobilization of metals and improving the soil quality. BC addition in highly acidic mine waste amplified the phytoremediation efficiency and mitigates abiotic oxidative stress on plants (MTI 84 % to >100 %). BC aided palmarosa plantation shifted the soil from high-risk assessment code (RAC) to low RAC for vegetation. Biochar amendments along with palmarosa plantation offer a sustainable technology for phytostabilization of highly acidic mine waste along with the production of industrially important essential oil.

Arundo donax L. stem-derived biochar increases As and Sb toxicities from nonferrous metal mine tailings

Toxic metal(loid)s released from tailing residues of mining operations have become a global issue with regard to environmental impacts. Biochar derived from the agriculture waste is considered as a cost-effective and stable material, which could be applied for remediation of sites contaminated with toxic metal(loid)s. In the present study, tailings were amended for 90 days with increasing concentrations of Arundo donax L. stem-derived biochar (ASBC; at 0, 1, 3, and 5%). The 7-day wheat seed germination toxicity test was then used to assess the bioavailability of toxicants in aqueous leachates of the biochar-amended tailing samples. Concentrations of As, Cd, Cu, Pb, and Sb in leachates and the Community Bureau of Reference chemical fractions were determined using ICP-OES. The results indicated that tailing leachates were phytotoxic, an effect that was partially decreased due to increasing concentrations of ASBC, with maximum effects (∼47% of tailing phytotoxicity) occurring at 3% ASBC. Results of further fractionation analyses indicated that increasing concentrations of ASBC amendment decreased the mobile fractions of Cd, Cu, and Pb in tailing samples, but increased the mobilities of As and Sb. A novel approach using the relative toxicity index (= sum of toxicities of individual potentially toxic elements) indicated that the toxicity of the tailings decreased when As was not present, since As decreased the biochar-reduced toxicity. Our results suggest that the ability of using biochar to decrease toxicity in tailings (by sequestration of cationic metals such as Cd, Cu, and Pb) is limited by its inability to immobilize oxyanionic metalloids such as As and Sb.

Application of wood biochar in polluted soils stabilized the toxic metals and enhanced wheat (Triticum aestivum) growth and soil enzymatic activity

Biochar is a stable carbonaceous by-product of pyrolysis and can be used for toxic metals (TMs) retention in polluted soil. Wheat (Triticum aestivum) was grown in three polluted soils collected from Chenzhou (CZ), Tongguan (TG) and Fengxian (FX), China. Wood biochar (WBC) was applied at 0, 0.5, 1.0 and 2.0% to each pot filled with 2 kg polluted soil. The results showed that WBC was efficient to alter soil pH and electrical conductivity (EC). The changes in soil pH and EC had a direct relationship with the immobilization and phytostabilization of TMs in the three soils. The bioavailable TMs (Zn, Pb, Cd, and Cu) were reduced in the soil after WBC amendments due to ion exchange, precipitates of metal-carbonates and metal-phosphates, and chemisorption on WBC surface. The reduction in the bioavailable TMs content also resulted in the diminution in TMs shoot uptake in wheat. Similarly, the TMs uptake in wheat root were also reduced as a result of WBC application. The reduction in bioavailable TMs and the release of essential nutrients and base cations from the WBC also increased the wheat shoot and root dry biomasses production. The application of WBC in polluted soil also improved soil health and the urease and β-glucosidase enzymes were also enhanced. The results concluded that WBC was efficient to reduce the bioavailability of TMs and shoot and root uptake, improved wheat dry biomasses production and soil enzymatic activities in industrial and smelter/mines polluted soils.

Biochar-mediated sequestration of Pb and Cd leads to enhanced productivity in Mentha arvensis

Immobilization of cadmium (Cd) and lead (Pb) along with the alleviation of their phytotoxicity in Mentha arvensis by biochar was examined in this investigation. A greenhouse experiment was executed to evaluate the effect of biochar (BC) amended Cd and Pb spiked soil on their immobilization and uptake, plant growth, photosynthetic attributes (total chlorophyll, photosynthetic rate, transpiration rate, and stomatal activity) and oxidative enzymes (guaiacol peroxidase: POD; catalase: CAT and superoxide dismutase: SOD). In the present study, the photosynthetic attributes showed that BC significantly improved the total chlorophyll, photosynthetic, transpiration rates, and stomatal activity in the plants. The incorporation of BC in soil increase the Pb and Cd tolerance in M. arvensis vis-à-vis improved the biomass yield and nutrient intake. In addition, biochar has also reduced the POD, CAT, and SOD in the plant as well as improved the soil pH and enzymatic activities. Overall, BC immobilized the Cd and Pb in soil by providing the binding site to the metals and reduced the phytotoxicity in M. arvensis. However, large-scale field trials of BC are required for safe cultivation of M. arvensis which is known for its phytopharmaceuticals importance.

Remediation of an acidic mine spoil: Miscanthus biochar and lime amendment affects metal availability, plant growth, and soil enzyme activity

Biochar may be a tool for mine spoil remediation; however, its mechanisms for achieving this goal remain unclear. In this study, Miscanthus (Miscanthus giganteus) biochar was evaluated for its ability to reclaim acidic mine spoils (pH < 3) through reducing metal availability, improving soil microbial enzymatic activity, and initial growth of grass seedlings. Biochar was applied at 0, 1, 2.5 and 5% (w/w) along with lime/no lime and fertilizer additions. Blue Wildrye (Elymus glaucus cv. 'Elkton') was planted and later the shoots and roots were collected and metal concentrations determined. Afterwards, each pot was leached with deionized water, and the leachate analyzed for pH, electrical conductivity (EC), dissolved organic carbon (DOC) and soluble metal concentrations. After drying, the spoil was extracted with 0.01 M CaCl₂ and Mehlich 3 (M3) to determine extractable Al, Cu, and Zn concentrations. Additionally, microbial activity was measured using a fluorescent β-glucosidase and N-acetyl-β-d-glucosaminidase assay. Spoil treated with lime and biochar had significantly greater pH and EC values. Significantly greater β-glucosidase activity occurred only in the 5% biochar plus lime treatment, while N-acetyl-β-d-glucosaminidase activities were not altered. Metal concentrations in rye shoot and roots were mixed. Lime additions significantly reduced extractable metal concentrations. Increasing biochar rates alone significantly reduced leachate DOC concentrations, and subsequently reduced leachable metal concentrations. Surprisingly, miscanthus biochar, by itself, was limited at mitigation, but when combined with lime, the combination was capable of further reducing extractable metal concentrations and improving β-glucosidase enzyme activity.

Review on utilization of biochar for metal-contaminated soil and sediment remediation

Biochar is a carbon-neutral or even carbon-negative material produced through thermal decomposition of plant- and animal-based biomass under oxygen-limited conditions. Recently, there has been an increasing interest in the application of biochar as an adsorbent, soil ameliorant and climate mitigation approach in many types of applications. Metal-contaminated soil remediation using biochar has been intensively investigated in small-scale and pilot-scale trials with obtained beneficial results and multifaceted effects. But so far, the study and application of biochar in contaminated sediment management has been very limited, and this is also a worldwide problem. Nonetheless, there is reason to believe that the same multiple benefits can also be realized with these sediments due to similar mechanisms for stabilizing contaminants. This paper provides a review on current biochar properties and its use as a sorbent/amendment for metal-contaminated soil/sediment remediation and its effect on plant growth, fauna habits as well as microorganism communities. In addition, the use of biochar as a potential strategy for contaminated sediment management is also discussed, especially as regards in-situ planning. Finally, we highlight the possibility of biochar application as an effective amendment and propose further research directions to ensure the safe and sustainable use of biochar as an amendment for remediation of contaminated soil and sediment.

Heavy metal immobilization and microbial community abundance by vegetable waste and pine cone biochar of agricultural soils

In order to determine the efficacy of vegetable waste and pine cone biochar for immobilization of metal/metalloid (lead and arsenic) and abundance of microbial community in different agricultural soils, we applied the biochar produced at two different temperatures to two contaminated soils. Biochar was produced by vegetable waste, pine cone, and their mixture (1:1 ww^-1) at 200 °C (torrefied biomass) and 500 °C (biochar). Contaminated soils were incubated with 5% (ww^-1) torrefied biomass or biochar. Sequential extraction, thermodynamic modeling, and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy were used to evaluate the metal immobilization. Microbial communities were characterized by microbial fatty acid profiles and microbial activity was assessed by dehydrogenase activity. Vegetable waste and the mixture of vegetable waste and pine cone biochar exhibited greater ability for Pb immobilization than pine cone biochar and three torrefied biomass, and vegetable waste biochar was found to be most effective. However, torrefied biomass was most effective in increasing both microbial community and dehydrogenase activity. This study confirms that vegetable waste could be a vital biomass to produce biochar to immobilize Pb, and increase the microbial communities and enzyme activity in soils. Biomass and pyrolytic temperature were not found to be effective in the immobilization of As in this study.

Negative interactive effects between biochar and phosphorus fertilization on phosphorus availability and plant yield in saline sodic soil

Little is known about the interactive effects between biochar application and phosphorus (P) fertilization on plant growth and P uptake. For this purpose, five wheat straw biochars (produced at 25°C, 300°C, 400°C, 500°C and 600°C for 4h) with equal P (36mgkg(-1)) amount, with and without additional P fertilization (100mgkg(-1)) were applied in a pot experiment to investigate the growth of Suaeda salsa and their uptake of P from biochar and P fertilization amended saline sodic soil. Soil P fractions, dry matter yield, and plant P concentrations were determined after harvesting 90days. Our results confirmed that relatively lower pyrolysis temperature (<400°C) biochar retained P availability and increased plant growth. The plant P concentration was significantly correlated with NaHCO3-Pi (P<0.05), and NaOH-Pi (P<0.1) during early incubation time (4days) for biochar amended soil. As revealed by statistical analysis, a significant (P<0.05) negative (antagonistic) interaction occurred between biochar and P fertilization on the biomass production and plant P concentration. For plant biomass, the effects size of biochar (B), P, and their interaction followed the order of B×P (0.819)>B (0.569)≈P (0.568) based on the partial Eta squared values whereas the order changed as P (0.782)>B (0.562)>B×P (0.515) for plant P concentration. When biochar and P fertilization applied together, phosphate precipitation/sorption reaction occurred in saline sodic soil which explained the decreased plant P availability and plant yield in saline sodic soil. The negative interaction effects between biochar and P fertilization indicated limited utility value of biochar application in saline sodic soil.